A bunch of us up here in the Pac NW just blended several batches into a Pinot Noir barrel and dumped in some brett and lacto. All batches were previously fermented before blending. We're a little shy on volume into the barrel so I was going to do up another batch. Should I let this batch ferment out for a few weeks then blend, or will the bugs benefit from having some fresh sugars added?

Our club did a pinot barrel flanders not long ago, and we ended up having to brew a couple of extra batches to top up the barrel over the 18 months it was in there. The angels must have really liked our brew
Anyway, go ahead and brew another batch, it'll be fine with fermenting it out then blending. There's plenty of sugars for the Brett. et al. to chew on already.

A bunch of my friends and I are going to be putting Flanders Red into a barrel next weekend. We are planning on adding 5 gallons of unfermented wort along with 55 gallons that has already fermented, just to make sure the brett/bacteria starts out happy. When I am normally doing sour beers I like to add the funk right at the start, the more sugar it gets to work through the happier I am with the resulting beer.

I agree though for top-ups over the next 12-18 months I would go with already fermented wort since it will be less likely to dramatically shift the conditions in the barrel. I know Lost Abbey keeps a barrel of their Mexican lager around to top-up all of their other barrel aged sour ales.